The preparation of N-alkyl(meth)acrylamides is known. The Ritter reaction has for a long time been employed for preparing amides from nitriles and substrates which can form carbenium ions, e.g. tertiary or secondary alcohols, in the presence of strong mineral acids. DE 3131096 utilizes this process. A disadvantage is that the acid, which also functions as solvent, has to be neutralized and large amounts of salts are thus obtained. Furthermore, the product contains interfering impurities such as methacrylamide.
DE 4027843 describes a continuous process for preparing N-substituted (meth)acrylamides by transesterification of alkyl esters of (meth)acrylic acid with aliphatic and aromatic amines. Although the reaction makes do without a catalyst, it is carried out under severe conditions at >150° and a pressure of about 160 bar.
The process claimed in EP 2358664 for preparing N-isopropylmethacrylamide by reacting methacrylic anhydride with isopropylamine requires, in the preferred variant, the presence of an inert organic solvent, which is associated with a purification requirement for recycling. In the solvent-free variant, large quantities of heat are evolved and can lead to discoloration of the product.
WO 2010/021956 discloses the preparation of N-alkyl(alkyl)acrylamides, likewise from the corresponding acid anhydride and alkylamine. The addition of the anhydride to the initially charged amine is claimed here. This process variant is likewise associated with strong evolution of heat, which can cause the discoloration cited above. In addition, there is only a narrow process window in which no severely troublesome lump formation occurs.
It is therefore an object of the present invention to provide an alternative process for preparing N-alkyl(meth)acrylamides which does not have the abovementioned disadvantages and, in particular, produces products in high yield and purity, with these being obtained under moderate reaction conditions, preferably from 1 to 10 bar and from 20 to 100° C., and in a problem-free manner.
These objects and also further objects which are not set forth in detail but can readily be deduced or derived from the introductory presentation of the prior art are achieved by a process having the features of claim 1. Advantageous modifications of the process of the invention are protected in the claims which refer back to claim 1.